The natural images, which reproduced the environments they were actually in, changed regularly — at night, high-res images of the moon were featured.

Kane says:

Thematically, the piece is ambiguously green. It appears to be replacing the
artificial with the natural, but it’s really just using technology to simulate a nature replacement. It’s also a form of “unvertising”—a campaign without a message. By removing the marketing message from the advertising space, we create an unexpected moment of introspection. People are allowed to interpret an image based on their own experience, and not necessarily with the singular focus of the advertiser’s intent.

There’s actually a great opportunity to rent unused billboard space at nominal or no cost from big providers. Clear Channel Outdoor Advertising, for instance, donates “millions of dollars’ worth of public service advertising (PSA) to nonprofit and governmental organizations for their use in communicating
information that positively affects the lives of those within our communities.”

My local radio station got free billboards, huge ones, on I-95. They only lasted a
month, which is the same time "Healing Tool" was up. As the company says, you need to be a nonprofit group, but penniless art galleries and radio stations qualify.

Here's a video that shows how cool the digital nature billboards looked: